Data collected in the NCPP are being analysed to determine primary and contributing roles of biological and environmental factors in mental retardation in a population of 40,000 children followed from the prenatal period to age 7. The incidence of severe retardation (0.5 percent) did not differ by ethnic group, but mild retardation was more frequent among blacks and Puerto Ricans (5 percent) than among whites (1 percent). Among the severely retarded children, three-fourths of the whites but only one-half of the blacks or Puerto Ricans had major neurological problems. Six to 14 percent of the mildly retarded children in each ethnic group also had major neurological problems. Perinatal anoxia was associated with mental retardation at all ages, and pychomotor test scores in infancy were good predictors of school-age retardation. Mild retardation was associated with lower socioeconomic status.